As is known in the art, a PiN diode includes an intrinsic semiconductor material, such as for example, silicon (Si) sandwiched between an n-doped region and a p-doped region. When the diode is reverse biased, the electric field fully depletes the intrinsic region. As a photodetector, the PiN diode is reverse biased. Under reverse bias, the diode ordinarily does not conduct (except for a small dark current or Is leakage). A photon entering the intrinsic region frees a carrier. The reverse bias field sweeps the carrier out of the region and creates a corresponding output current.
As is also known in the art, the noise performance of silicon diode (Si:PiN) photodetectors is limited by the surface leakage or dark current. Exposure to various environmental conditions such as ionizing radiation or humidity can further degrade the performance by increasing the surface leakage current. Present devices are unfortunately designed in a manner that maximizes the surface generated dark current by biasing the surface in depletion. The contribution is partially mitigated by the use of a channel stop to remove the potential minima away from the surface and by annealing to passivate interface states, but the result is still a device where the surface dark current limits the ultimate performance.
In the current art, these devices are made from single crystal Silicon. The surface of the crystal is oxidized to produce a SiO2 passivation layer. In the interface between the Silicon and the SiO2, there are inherent defects due to the mismatch between the two materials. At these defect sites there are unbonded Si orbitals that are electrically active. A normal Silicon process attempts to passivate these dangling orbitals by annealing in Hydrogen, which bonds to the defect site and reduces its electrical activity. Such Hydrogen passivation is never completely effective, so some small percent of the precursor defect sites remain electrically active where they contribute leakage current due to trapping/detrapping that occurs at the resulting recombination center.